1. Field of the Invention
The present invention relates to a nonvolatile magnetic memory device.
2. Description of the Related Art
Attendant on the drastic spread of personal small apparatuses, such as information communication apparatuses, particularly, mobile terminals, various semiconductor devices, such as memory elements or logic elements constituting the apparatuses, are desired to have higher performance, such as higher degree of integration, higher operating speed, lower power consumption, and the like. Particularly, nonvolatile memories are considered to be keenly desired in the ubiquitous computing age. Even in the cases of consumption or trouble in the power supply or in the cases of cutoff between a server and a network due to some disorder, the nonvolatile memory makes it possible to preserve and protect important information. In addition, while the recent portable apparatuses are designed to suppress power consumption as much as possible by putting unnecessary circuit blocks into the standby state, if a nonvolatile memory capable of functioning as both a high-speed work memory and a large-capacity storage memory can be realized, it is possible to eliminate the wastefulness in power consumption and memory. Furthermore, the “instant-on” function enabling an instantaneous start upon making the power supply can also become possible if a high-speed large-capacity nonvolatile memory can be realized.
Examples of the nonvolatile memory include flash memories using semiconductor materials, and ferroelectric nonvolatile semiconductor memories (FERAMs, Ferroelectric Random Access Memories) using ferroelectric materials. However, the flash memories have the defects that the write speed is on the order of microseconds, which is lower than desired. Meanwhile, the FERAM has a number of times of rewriting possible on the order of 1012 to 1014, which is too low for the FERAM to be used in place of SRAM or DRAM, and micro-processing of the ferroelectric material layer is difficult to carry out.
As a nonvolatile memory free of the above-described defects, the nonvolatile magnetic memory devices called MRAMs (Magnetic Random Access Memories) have come to be paid attention to. Of the MRAMs, the MRAM using the TMR (Tunnel Magnetoresistance) effect has come to be paid attention to with recent improvement of characteristics of TMR materials (for example, see Wang et al., “Feasibility of Ultra-Dense Spin-Tunneling Random Access Memory”, IEEE Transactions on Magnetics, Vol. 33, November 1997, pp. 4498-4512). The TMR type MRAM is simple in structure, promises easy scaling, and has a large number of times of rewriting possible because of the recording by rotation of the magnetic moment. Furthermore, with the TMR type MRAM, a very short access time is expected, and it is said that the TMR type MRAM has already come to be able to operate at a rate of 100 MHz (for example, see R. Scheuerlein et al., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, 2000 IEEE International Solid-State Circuits Conference Digest of Technical Papers, February 2000, pp.128-129).
A schematic, partially sectional view of the TMR type MRAM (hereinafter, simply referred to as MRAM) is shown in FIG. 18. The MRAM includes a magnetic tunneling junction element (also called an MTJ element) connected to a selection transistor TR including a MOSFET.
The MTJ element has a laminate structure of first ferromagnetic material layer 2051, a tunnel insulation film 2052, and a second ferromagnetic material layer. Specifically, the first ferromagnetic material layer 2051 has a two-layered structure of, for example, an antiferromagnetic material layer 2051A and a ferromagnetic material layer (also called a pinned layer or magnetization pinned layer 2051B), in this order from the lower side, and has a strong unidirectional magnetic anisotropy due to the exchange interaction between the two layers. The second ferromagnetic material layer whose magnetization direction can be rotated comparatively easily is also called a free layer or a recording layer. In the following description, the second ferromagnetic material layer may be called a recording layer 2053. The tunnel insulation film 2052 plays the roles of interrupting magnetic coupling between the recording layer 2053 and the magnetization pinned layer 2051B, and of passing a tunnel current. A bit line BL (second wiring) for connection between the MRAM and the MRAM is formed on an interlayer insulation layer 26. A cap layer 2061 provided between the bit line BL and the recording layer 2053 functions to prevent mutual diffusion between the atoms constituting the bit line BL and the recording layer 2053, to reduce contact resistance, and to prevent oxidation of the recording layer 2053. In FIG. 18, reference numeral 2041 denotes a first wiring connected to a lower surface of the antiferromagnetic material layer 2051A.
A write word line WWL is disposed below the MTJ element with a second lower insulation layer 24 interposed therebetween. The extension direction (first direction) of the write word line WWL and the extension direction (second direction) of the bit line BL (second wiring) are ordinarily orthogonal to each other.
The selection transistor TR is formed in a portion of a silicon semiconductor substrate 10 surrounded by an element isolation region 11, and is covered with a first lower insulation layer 21. A source/drain region 14B on one side is connected to the first wiring 2041 of the MTJ element through a contact hole 22 including a tungsten plug, a landing pad portion 23, and a contact hole 25 including a tungsten plug. A source/drain region 14A on the other side is connected to a sense line 16 through a tungsten plug 15. In FIG. 18, reference numeral 12 denotes a gate electrode (functioning as a so-called word line), and reference numeral 13 denotes a gate insulation film. In the MRAM array, the MRAM is arranged at each of the intersections (overlap regions) in the lattice including the bit lines BL and the write word lines WWL.
In writing data into the MRAM having the above-described configuration, a current in a positive or negative direction flows in the bit line BL, while a current in a fixed direction flows in the write word line WWL, and a composite magnetic field thus generated changes the magnetization direction of the recording layer 2053. Thus, “1” or “0” is recorded in the recording layer 2053.
Meanwhile, reading of data is conducted by setting the selection transistor TR into the on state, passing a current through the bit line BL, and detecting via the sense line 16 a change in the tunnel current due to the magnetoresistance effect. When the magnetization directions of the recording layer 2053 and the magnetization pinned layer 2051B are equal, a low resistance result is obtained (this state is made to be “0”, for example), and when the magnetization directions of the recording layer 2053 and the magnetization pinned layer 2051B are anti-parallel, a high resistance result is obtained (this state is made to be “1”, for example).
In the MRAM, in order to stably hold recorded information, the recording layer 2053 in which information is to be recorded needs to have a uniform coercive force. Meanwhile, in order to rewrite recorded information, a predetermined amount of current needs to pass through the bit line BL. However, with the miniaturization of the MRAM, the bit line BL is made finer, and accordingly a sufficient amount of current is difficult to pass through the bit line BL. For this reason, as a structure allowing magnetization inversion with a smaller amount of current, a spin injection type magnetoresistance effect element applying magnetization inversion by spin injection has come to be paid attention to (for example, see JP-A-2003-17782) Magnetization inversion by spin injection is a phenomenon that an electron spin-polarized through a magnetic material is injected into other magnetic material, and magnetization inversion occurs in other magnetic material.
A conceptual view of a spin injection type magnetoresistance effect element is shown in FIG. 2A. The spin injection type magnetoresistance effect element includes a magnetoresistance effect multi-layer film including a multi-layer film having a GMR (Giant Magnetoresistance) effect or a TMR effect. The magnetoresistance effect multi-layer film is interposed between two wirings 41 and 42. Specifically, the magnetoresistance effect multi-layer film includes a recording layer (also called a magnetization inversion layer or free layer) 53 having a function of recording information and a magnetization reference layer (also called a pinned layer) 51 having a pinned magnetization direction and serving as a spin filter with a nonmagnetic material film 52 interposed therebetween. A current flows in a direction perpendicular to the film surface (see FIG. 2A). A schematic plan view of the recording layer 53 is shown in FIG. 2B. The size of the recording layer 53 depends on the type or thickness of a magnetic material forming the recording layer 53. The size of the recording layer 53 is about 200 nm or less in order to promote single magnetic domain formation and reduce a critical current Ic of spin-injection magnetization inversion. The recording layer 53 may have two or more magnetization directions (for example, two lateral directions indicated by arrows in FIG. 2A, that is, a first direction and a second direction) by an appropriate magnetic anisotropy, and each of the magnetization directions corresponds to information to be recorded. In the example of FIG. 2B, the recording layer 53 is a long elliptic in a planar shape so as to be provided with a magnetic shape anisotropy. That is, the recording layer 53 has an easy magnetization axis parallel to the first direction and the second direction, and a hard magnetization axis orthogonal to the easy magnetization axis. The length of the recording layer 53 along the easy magnetization axis is longer than that of the recording layer 53 along the hard magnetization axis.
FIG. 19 shows the relationship between a memory cell size and a write current. In FIG. 19, the right vertical axis represents the memory cell size (F2), the left vertical axis represents a write current, and the horizontal axis represents the size of a short side of an MTJ element. As will be apparent from FIG. 19, in the spin-injection magnetization inversion type, as the element size is reduced, the write current also decreases. When the cell size is the same as that of an embedded DRAM, the write current becomes small so as to be 100 μA. Meanwhile, in a known MRAM, as the element size is reduced, the write current significantly increases. When the cell size is the same as that of a 6-transistor type SRAM (6-TSRAM), the write current is about 1 mA.
In such a spin injection type magnetoresistance effect element, the device structure can be simplified, as compared with the MRAM. In addition, since magnetization inversion by spin injection is used, as described above, even if the element is miniaturized, the write current does not increase, as compared with the MRAM in which magnetization inversion is produced by an external magnetic field.